Thermoregulation

Thermoregulation

The processes by which many animals actively maintain the temperature of part or all of their body within a specified range in order to stabilize or optimize temperature-sensitive physiological processes. Body temperatures of normally active animals may range from 32 to 115°F (0 to 46°C) or more, but the tolerable range for any one species is much narrower.

Animals are commonly classified as warm-blooded or cold-blooded. When the temperature of the environment varies, the body temperature of a warm-blooded or homeothermic animal remains high and constant, while that of a cold-blooded or poikilothermic animal is low and variable. However, supposedly cold-blooded reptiles and insects, when active, may regulate body temperatures within 2–4°F (1–2°C) of a species-specific value. Supposedly warm-blooded mammals and birds may allow their temperature to drop to 37–68°F (5–20°C) during hibernation or torpor. Further, optimal temperature varies with organ, time of day, and circumstance. Thus, this classification is often misleading.

A better classification is based on the principal source of heat used for thermoregulation. Endotherms (birds, mammals) use heat generated from food energy. Ectotherms (invertebrates, fish, amphibians, reptiles) use heat from environmental sources. This classification also has limitations, however. For example, endotherms routinely use external heat sources to minimize the food cost of thermoregulation, and some ectotherms use food energy for thermoregulation. SeePhysiological ecology (animal)

Behavior is the most obviously active form of thermoregulation. Most animals are mobile, sensitive to their environment, and capable of complex behaviors. The simplest thermoregulatory behavior consists of moving to a favorable location. Operative temperature may also be altered by changing posture in one place. Lizards face the sun to minimize the area exposed to solar heating, or orient broadside to maximize it, and some ground squirrels use their tail as a sunshade. Some reptiles and amphibians also expand or contract pigmented cells in their skin to increase or decrease solar heating. SeeChromatophore

Evaporation is an effective means of cooling the body. Evaporation from the respiratory mucous membranes is the most common mechanism. Evaporation from the mucous membranes cools the nose during inhalation. During exhalation, water vapor condenses on the cool nasal membranes and is recovered. Evaporation can be greatly increased by exhaling from the mouth to prevent condensation. Additional increases in evaporation result from panting, that is, rapid breathing at the resonant frequency of the respiratory system. Evaporation from the eyes and the mucous membranes of the mouth and tongue is another source of cooling. Water is also evaporated from the skin of all animals, and can be varied for thermoregulation. Some desert frogs control evaporation by spreading an oily material on the skin. Reptiles, birds, and mammals have relatively impermeable skins, but evaporation can be increased by various means. The sweat glands in the skin of mammals are particularly effective and are one of the few purely thermoregulatory organs known. SeeSkin

Changes in circulation can be used to regulate heat flow. Countercurrent heat exchange is used to regulate heat flow to particular parts of the body while maintaining oxygen supply. Large vessels may be divided into intermingled masses of small vessels to maximize heat exchange, forming an organ called a rete. However, retes can be bypassed by alternative circulation paths to regulate heat flow. Many animals living in warm environments have a rete that regulates brain temperature by cooling the arterial blood supply to the brain with blood draining from the nasal membranes, eyes, ears, or horns. SeeCardiovascular system

Heat exchange with the environment is limited by the fur of mammals, feathers of birds, and furlike scales or setae of insects. Erection or compression of this insulation varies heat flow. Insulation thickness varies over the body to exploit variations in local operative temperature. Thermal windows are thinly insulated areas that are either shaded (abdomen of mammals, axilla of birds and mammals) or of small size (ears, face, legs) so that solar heating is minimized.

The oxidation of foodstuffs within the metabolic pathways of the body releases as much heat as if it were burned. Basal metabolism is the energy use rate of a fasting animal at rest. Activity, digestion, and thermoregulation increase metabolism above the basal rate. Endothermy is the utilization of metabolic heat for thermoregulation. Birds and mammals are typical examples, but significant endothermy also occurs in large salt-water fish, large reptiles, and large flying insects. Endotherms regulate only the temperature of the body core, that is, the brain, heart, and lungs. The heat production of these metabolically active organs is often supplemented with heat produced in muscles. Heat produced as a by-product of activity may substitute partially for thermoregulatory heat production, and imposes no thermoregulatory energy cost. In contrast, shivering produces heat only for thermoregulation and results in an extra cost. Some animals have specialized heater organs for nonshivering thermogenesis, which is more efficient than shivering. Brown adipose tissue is a fatty tissue with a high density of mitochondria. It is found in the thorax of mammals, especially newborns and hibernators, and it warms the body core efficiently. SeeMetabolism

The variety of mechanisms used in thermoregulation indicates a corresponding complexity in neural control. Temperature sensors distributed over the skin respond nearly immediately to changes in the environment and provide the major input. Nearly all parts of the central nervous system also respond to local thermal stimulation. These peripheral and central thermal inputs are integrated at a series of centers beginning in the spinal cord. This series clearly extends to the cerebral cortex, as a learning period is required before behavioral thermoregulation reaches maximum precision. Various components respond to the rate of temperature change as well as the difference between preferred and actual temperature. The neuroendocrine system then regulates metabolic heat production, the sympathetic nervous system controls blood flow, and the cerebral cortex controls behavioral thermoregulation. SeeEndocrine system (vertebrate), Homeostasis, Nervous system (vertebrate)

Thermoregulation

(also heat regulation), the ability of man, mammals, and birds to maintain the temperature of the brain and internal organs within certain narrow limits, despite marked variations in the ambient temperature and in their own thermogenesis. The internal temperature of the body is maintained at a relatively constant level through the process of self-regulation. The body temperature is kept constant by thermogenesis, which is frequently called chemical thermoregulation, and heat elimination, known as physical thermoregulation.

The thermoregulatory system comprises a thermoregulatory center situated in the hypothalamus and numerous heat-sensitive nerve cells in various parts of the central nervous system, from the cerebral cortex to the spinal cord. The system also includes thermoreceptors of the viscera, mucous membranes, and skin, with corresponding nerve conducting pathways, as well as efferent nerve pathways and effector organs—cutaneous blood vessels, endocrine glands, sweat glands, and skeletal muscles.

When the body is in danger of hyperpyrexia, typical reactions are the dilation of cutaneous blood vessels, increased perspiration (or panting in animals that do not perspire), and increased heat elimination. When the body is in danger of cooling, the cutaneous blood vessels constrict, the hairs or feathers stand on end (piloerection), and heat elimination is restricted, while thermogenesis increases. Thus the body maintains a balance between thermogenesis and heat elimination under different temperatures. Any deviation of the average temperature in the internal regions of the body, blood, muscles, and skin from the established level intensifies the flow of impulses from the heat-sensitive nerve cells and thermoreceptors. The impulses reach the thermoregulatory center in the hypothalamus, and the thermoregulatory center sends a control signal to the thermoregulatory effector organs.

Thermoregulation is controlled by the higher divisions of the brain, the cerebral cortex in particular. The body’s general sensitivity to temperature results in complex reactions of behavioral thermoregulation, for example, the active avoidance of excessive heat or cold, building of burrows, nests, and other shelters by animals, and an animal’s changing the area of its exposed body surface by rolling itself into a ball in response to cold.

The effectiveness of thermoregulation depends on various factors. If the ambient temperature drops substantially or thermogenesis changes sharply, the temperature of the brain and internal organs of man and various animals may deviate from the usual level by 0.2°–0.3°C to 1°–2°C or more. Thermoregulatory mechanisms are not equally developed among various organisms. For example, perspiration is peculiar only to man, monkeys, and perissodactyls (odd-toed ungulates). In other homeothermic animals, however, panting is the most efficient means of heat elimination. The capacity for increasing thermogenesis is most pronounced in birds, rodents, and some other animals.

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